1. Field of the Invention
The present invention relates to a polishing apparatus and method, and more particularly to a polishing apparatus and method for polishing a workpiece such as a semiconductor wafer to a flat finish by bringing a surface of the workpiece into contact with a polishing surface on a turntable.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical Polishing (CMP) in which the semiconductor wafers are chemically and mechanically polished while supplying a polishing liquid containing certain components and comprising abrasive particles and a chemical solution such as an alkaline solution.
In the polishing apparatus for polishing a surface of a semiconductor wafer, especially a device pattern on the upper surface of a semiconductor wafer, to a flat finish, as a polishing cloth attached to a turntable, a polishing cloth made of a nonwoven fabric or polyurethane foam is employed. Further, an impregnated pad, in which abrasive particles are impregnated into a pad, is also used.
After the semiconductor wafer is contacted with the polishing cloth and polished by rotating the turntable and the top ring which holds the semiconductor wafer, the polishing capability of the polishing cloth is deteriorated due to a deposit of abrasive particles and ground-off particles of the semiconductor material, and due to change in the characteristics of the polishing cloth. Therefore, if the same polishing cloth is used to repeatedly polish semiconductor wafers, the polishing rate of the polishing apparatus is lowered, and the polished semiconductor wafers tend to suffer polishing irregularities. Therefore, it has been customary to condition the polishing cloth according to a process called xe2x80x9cdressingxe2x80x9d for recovering the surface of the polishing cloth before, or after, or during polishing.
In the dressing process, a dresser element comprising a dresser plate to which diamond particles are attached by electrodeposition is brought into contact with the polishing cloth and rubs the polishing cloth between the polishing processes.
However, in the above conventional method, the polishing rate is lowered immediately after dressing of the polishing cloth is conducted, and hence the polishing condition is unstable. During polishing, the polishing cloth is kept at a certain temperature higher than room temperature due to heat balance between generation of heat caused by friction between the semiconductor wafer and the polishing cloth, and cooling effect caused by the polishing liquid. However, it is considered that after dressing, as shown in FIG. 6, the temperature of the polishing cloth is lowered to substantially the same temperature as the dresser element, and hence it takes time until the temperature of the polishing cloth increases by performing a polishing operation and reaches a steady state. In FIG. 6, the horizontal axis represents the time and the vertical axis represents the temperature of the polishing cloth.
Specifically, when dressing is performed by the conventional dressing apparatus, the heating value Qd is small because the pressing force in the dressing process is smaller than that in the polishing process. Therefore, as shown in FIG. 7, the temperature of the polishing surface is lowered from t1 to t2 due to heat conduction between the polishing cloth and the dresser element which is kept at a temperature in the polishing apparatus or room temperature (normal temperature) in a clean room (or a room in which the polishing apparatus is installed). Therefore, while the workpiece (semiconductor wafer) is being polished after the dressing process, the temperature of the polishing surface increases gradually and reaches the steady state again. In this manner, in the conventional method, the temperature of the polishing surface cannot be kept constant during polishing. Particularly, in the chemical mechanical polishing process in which chemical reaction is caused on a polished surface of the workpiece (semiconductor wafer) by utilizing a chemical solution such as an alkali solution in the polishing liquid, the reaction rate is changed as the temperature is changed. As a result, the polishing rate is greatly changed, and control of thickness of the polished layer (or film) becomes difficult.
It is therefore an object of the present invention to provide a polishing apparatus and method which can polish a workpiece such as a semiconductor wafer to a surface having a high degree of flatness and control a film thickness on the workpiece to a desired value by conducting a polishing process in a stable condition after dressing.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a workpiece, comprising: a turntable having a polishing surface thereon; a top ring for holding a workpiece and pressing the workpiece against the polishing surface; a dressing apparatus having a dresser element for dressing the polishing surface by bringing the dresser element in contact with the polishing surface; and a temperature control device for controlling the temperature of the dresser element before dressing and/or during dressing.
With the above arrangement, the temperature of the dresser element is controlled by the temperature control device before dressing and/or during dressing so as to be equal to or higher than that of the polishing surface of the polishing tool (turntable) which is in a steady state during polishing. That is, the polishing surface during dressing can be kept at the same temperature as that of the polishing surface during polishing, which is in a steady state. The heat conduction during dressing is similar to normal heat conduction because the heating value is small, and therefore the temperature of the polishing surface can be adjusted by controlling the temperature of the dresser element.
Specifically, conventionally, the polishing surface shows temperature change shown in FIG. 6. However, by employing the dressing apparatus of the present invention, the polishing surface shows temperature change shown in FIG. 5, and hence the workpiece can be polished at a constant temperature at all times. Therefore, the reaction rate in the polishing process can be kept constant, and polishing of the workpiece can be conducted in a stable condition.
Further, by controlling the temperature of the dresser element, the dressing process can be stabilized, and the polishing surface can be better conditioned.
The temperature control of the dresser element may be conducted before dressing and/or during dressing. Further, the temperature control may be conducted in a feedback control loop while detecting the temperature of the dresser element. The temperature control by the temperature control device is not limited to heating, and may include cooling. For example, if polishing of the workpiece is performed at a temperature equal to or lower than the temperature in the polishing apparatus, then the temperature of the dresser element is controlled so as to be substantially equal to the temperature of the polishing surface. The polishing surface may comprise a polishing cloth, or an abrading plate (fixed abrasive plate).
According to a second aspect of the present invention, there is provided a polishing method for polishing a surface of a workpiece, comprising: polishing a workpiece by bringing a workpiece in contact with a polishing surface on a turntable; dressing the polishing surface by bringing a dresser element of a dressing apparatus in contact with the polishing surface; and controlling the temperature of the dresser element by a temperature control device before dressing and/or during dressing.
According to a third aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface on a turntable, comprising: a dresser element for dressing a polishing surface by bringing the dresser element in contact with the polishing surface; and a temperature control device for controlling the temperature of the dresser element before dressing and/or during dressing.
In a preferred aspect, the temperature of the polishing cloth is detected directly or indirectly, and the temperature of the dresser element is controlled on the basis of the detected temperature of the polishing cloth.
In this case, a remote sensor such as a thermocouple thermometer or a radiation thermometer is used to detect the temperature of the polishing surface directly. In order to detect the temperature of the polishing surface indirectly, in an example, the relationship between the torque of the turntable motor for rotating the turntable and the temperature of the polishing cloth is found in advance, and the temperature of the polishing surface during polishing may be estimated by detecting the torque of the turntable motor and utilizing the above predetermined relationship.
In a preferred aspect, the temperature of the dresser element is controlled so that the temperature of the dresser element is substantially equal to the temperature of the polishing surface, which is in a steady state, during polishing.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, which illustrate preferred embodiments of the present invention by way of example.